Abstract:

The invention is related to the method for production of diamond electrode
with improved stability for use in electrochemical reaction. The method
concerns to the production of diamond electrodes where the diamond layer
is composed of small sized grain, avoiding the delamination problems
found in conventional diamond electrodes.

Claims:

1. A method for producing diamond electrode by coating a substrate
material by CVD process;said diamond electrode having a single and
homogeneous layer composed of a poly-crystalline and conductive diamond
with grain size lower than one micrometer;said layer having a Raman
quality higher than 50%;wherein the said layer is produced by controlling
the CVDat pressure lower than 20 mBar; andat methane concentration lower
than 2%.

2. The method for producing the diamond electrode according to the claim
1, further comprises a pretreatment step before CVD process,said step
comprising a procedure where nano-sized diamonds are used as seed
crystals.

3. The method for producing the diamond electrode according to the claim
1, wherein the diamond layer has a thickness of at least one micrometer.

4. The method for producing the diamond electrode according to the claim
1, wherein the diamond layer has a boron doping level lower than 1500
ppm.

5. The method for producing the diamond electrode according to the claim
1, wherein the CVD process comprises a hot-filament CVD process, where
the filament are disposed vertically.

Description:

TECHNICAL FIELD

[0001]The present invention concerns to a method for producing diamond
electrodes with improved stabilities for use in aqueous media. The
diamond electrode with improved stabilities can be used, in treatment of
industrial and urban wastewater, in disinfection of freshwater and
seawater, in electrochemical organic/inorganic synthesis and in
electrochemical sensor for detection of dilutes compounds in the water or
other application of electrode in aqueous media.

BACKGROUND ART

[0002]Diamond is known to be one of hardest materials which allow its
application in tools for machining mechanical components such as drills
and grinders. Besides these physical properties of diamond, in last two
decades, peculiar electrochemical properties of diamond has been found
when used as electrodes. Diamond electrodes show a large thermodynamic
windows and exhibit efficient production of OH radical from the water.
Peculiar electrochemical properties of diamond have been working as an
incentive for development of new application such as a sensor and
electrodes for water treatment process or electrochemical synthesis.
These diamond electrodes are produced by coating a conductive substrate;
such as silicon, graphite, or metal like Niobium, Titanium, Tungsten,
Molybdenum, Tantalum, or other electrically conductive and high
temperature resistant material; with a layer of conductive diamond by
Chemical Vapor Deposition (CVD) process. Natural diamonds are
electrically insulating material but when the diamond are doped with a
P-type dopant or N-type dopant; N-type semi-conductor or P-type
semiconductor diamond layer can be fabricated by the CVD process.
Basically two types of CVD process are commonly used for coating the
substrate material: hot filament CVD (HF-CVD) or microwave-CVD (MW-CVD).
Between these two CVD methods, HF-CVD has been advantageously used for
coating large area electrodes. Large area coating can be performed by the
HF-CVD disposing an array of filament inside the CVD chamber. In the CVD
coating process, the substrate material are coated with a
poly-crystalline diamond layer when the substrate are heated to around
700-900° C. in presence of a hydrogen radical, carbon and dopant
source. The hydrogen radical source is usually hydrogen gas activated by
hot filament or plasma generated by the microwave; depending on the type
of the CVD. The hydrogen radical are produced by activating the hydrogen
gas by the hot filament kept at around 1,800 to 2,400° C. or
plasma at temperature 1,500 to 6000° C. Several carbon sources can
be used for this CVD coating but one commonly used compound is methane
gas kept at concentration around 0.5 to 10 volume percent (hereinafter %)
in hydrogen atmosphere. During the CVD process, sp3 carbons (diamond
carbon) and sp2 carbons (non-diamond carbon) are formed at the same
time. However, because the hydrogen radicals react preferentially with
sp2 carbons, the deposition of sp3 carbon over the substrate
material can take place inside of CVD chamber This deposition of sp3
carbon allows the growing of diamond crystals over the substrate
material. Several dopant source can be used depending on the type of
semiconductor diamond in interest, but one of the most common dopant is
Boron in form of diborane, tri-methyl boron or boron dioxide at
concentration of less than one volume percent in hydrogen atmosphere; for
the production of p-type conductor diamond layer.

[0003]Prior arts have disclosed methods for producing diamond electrodes
using the CVD process.

[0004]JP 2004-231983 (A) discloses a method for production of diamond
electrodes in which the electrodes comprise two layers of diamond; being
at least one layer of conductive diamond. Furthermore, this patent
discloses the manufacturing of diamond layer with different grains sizes
to improving the stability during electrochemical process. Changes in
methane concentration and substrate temperature during the CVD process
are suggested for controlling the grain size in different diamond layers.
The disclosed CVD coating pressure is ranging from 90 mBar to 160 mBar
(9-16 kPa).

[0005]JP 2003-147527(A) discloses another method for production of diamond
electrode by coating graphite substrates. The disclosed electrode has an
intermediate diamond layer with grain size lower than 10 nm. Also here,
for production of such fine grain size layer, the increase in the methane
concentration to values higher than 5% is proposed. The disclosed CVD
coating pressure is ranging from 67 to 75 mBar (6.7-7.5 kPa)

[0006]The usual CVD condition for production of diamond electrodes in the
majority of previous art can be summarized as bellow;

[0007]Methane concentration: 0.5-10%

[0008]CVD chamber pressure: 26-160 mBar (2.6-16 kPa)

[0009]CVD Atmosphere: Hydrogen

[0010]Features in the previous art focus on the improvement of diamond
stability by producing small grains diamond which can works as a barrier
for the electrolyte solution. The methodology to obtain a diamond layer
with small grain size is by controlling the methane concentration to
value higher than one percent or by controlling the temperature of the
substrate to low value.

[0011]In the previous application of the present inventors, WO2005/113448
A1, diamond electrode with improved stability was also disclosed in which
the electrode is composed of alternated layer with different grain sizes.
The layer was produced at 7 mBar and the technique to change the grain
size here was also changing the methane concentration between 1 and 2%.
Also the variation of CVD temperature is disclosed as a method to change
the grain size.

[0012]This invention relates to an improvement of the prior application
and prior art.

DISCLOSURE OF THE INVENTION

[0013]Diamond electrodes exhibits fascinating properties for example in
removing COD component in aqueous medium due to the huge amount of OH
radical produced in its surface. Such performance can not be achieved by
other conventional electrode such as graphite electrode, platinum or
other noble metal electrode like DSE (dimensionally stable electrode).
Although diamond electrode performance being very promising; the
industrial applicability has been strongly limited by its poor stability
when used in almost all types of electrolytic reaction in aqueous medium.
Current DSE used in the Chloro-Alkali industry has stability longer than
several years, but in comparison, the stability of actual diamond
electrodes are extremely short. It is known by the status of art that
when electrode with large diamond layer thickness; for example higher
than 50 micrometer is used, such life time requirement can be cleared.
Due to the difference in thermal expansion coefficient, such high
thickness diamond can be coated only in silicon substrates. When Niobium
or other metals are used as the substrate, there is a bending of
electrode due to the high temperature coating. Also there is a problem of
remaining stress in the coating, which usually contributes to decrease
the life time of electrode. High thickness diamond coating are possible
over the silicon substrate, but the CVD coating cost of such large
thickness diamond layer will become prohibitive for a commercial use.
Therefore, the scope of present invention is to provide a diamond
electrode and a method for production of diamond electrode with lower
production cost and improved stability.

[0014]Basically, two mechanisms strongly contribute for the fail or
break-down of diamond electrode during the electrolytic reaction: etching
and delamination of diamond layer. On one hand, the etching of the
diamond layer is a process that slowly deteriorates the diamond
electrodes. It is thought that the etching mechanism proceeds by a
chemical oxidation where the electrochemically produced OH radicals
formed in the diamond surface attacks the diamond layer itself. These OH
radical are the oxidant that promote the COD compound oxidation during
wastewater treatment or kill the microorganisms during the water
disinfection. The performance of diamond electrodes are attributed to the
production of this strong oxidant, but at the same time, this strong
oxidant works to corrode the diamond layer itself. Microscopically, the
etching of the diamond layer proceeds by the attack of OH radical to the
parts in which the diamond layer has weak chemical stability. Such weak
parts are twins defects in the diamond grains, specific crystal
orientation where the dopant or sp2 carbon tends to concentrate and
inter-granular region. Macroscopically, it can be said that the etching
proceed homogeneously in the whole surface of diamond electrode but in a
very slow speed.

[0015]On the other hand, the delamination of diamond layer is a mechanism
that rapidly causes the fail of diamond electrode. The delamination of
the diamond layer proceeds by the detachment of diamond layer from the
substrate material. Macroscopically, the delamination starts in a
heterogeneous way in the diamond electrode surface, but quickly
propagates to the whole surface. The delamination mainly happens due to
the corrosion of interlayer, which is a middle layer that bonds the
diamond layer and the substrate material. This bonding interlayer is
formed at the beginning of the CVD coating process and its chemical
composition varies depending on the used substrate material in the CVD
coating process. The interlayer composition will be, for example, silicon
carbide, titanium carbide or niobium carbide when the used substrate is
silicon, titanium or niobium, respectively. These carbides interlayer has
poor stability against the electrochemical attack of electrolyte solution
during the electrolytic process. The diamond layer over this carbide
interlayer has to work as a barrier against the electrolyte solution
during the electrolytic process to avoid this delamination. However,
defects in the diamond layer surface such as pinholes, or the
inter-granular etching of poly-crystalline layer allows the penetration
of electrolyte solution starting the delamination FIG. 1 shows a
schematic illustration of delamination mechanism originated by pinhole.
These pinholes tends to appear in the layer when the condition for CVD
coating is likely to form large diamond grains. There is almost a
straight path for the electrolyte solution to reach the carbide
interlayer when the grains are bigger. The sp2 carbons and the
dopant, which causes the decrease of chemical stability of the diamond
layer, tend to accumulate in the grain boundary region. Even in the case
that there is not a pinhole at the beginning, because the inter-granular
regions are preferentially etched, the pinhole are easily formed during
the electrolytic process. Despite the nuclei of diamond grains being
resistant against the electrochemical etching, this sub-surface migration
of the electrolyte causes the delamination of diamond layer. This
penetration of electrolyte solution through pinholes or inter-granular
etching is illustrated in FIG. 1a and this happens mainly when the
diamond layer is comprised by large poly-crystalline diamonds.
Specifically speaking, this penetration of electrolyte solution through
the diamond layer easily happens when the diamond grain size is larger
than one micrometer. During the coating process in the CVD chamber, the
diamond crystal tends to grow in a columnar structure, which means, the
grain has longitudinal dimension larger than width dimension. More
specifically speaking, this problem of electrolyte penetration can happen
when the width dimension of diamond grain is larger than one micrometer.
Once a delaminated area appears at the electrode surface, the electrolyte
solution easily attacks the carbide interlayer in the vicinity of
delaminated area, propagating the delamination to the whole electrode
surface. This propagation of delaminated area by the sub-surface
migration of electrolyte solution is illustrated in FIG. 1b.

[0016]FIG. 2 shows a cross section of another embodiment of diamond
electrode with different structure, which is one of preferred embodiment
of present invention. In this embodiment of diamond electrode, the
diamond layer is composed of small grains with size between 0.1-800 nm in
width; preferable in the range between 1-500 nm; more preferable in the
range between 1-300 nm. Because of the small grain size, the diamond
layer is compact and has a minute structure which avoids the pinhole or
cavity. This structure has the advantage in blocking the penetration of
electrolyte solution. Furthermore, even when the inter-granular region of
the layer are etched and forms a path for the penetration of electrolyte
solution through the diamond layer, this path is not a straight path. Due
to the small grain size, the path for the penetration of electrolyte
solution becomes a labyrinthine path and this path gain time until the
electrolyte solution reaches to the carbide interlayer. Then, such
diamond electrode structure can clearly extend the life time and improve
the stability of diamond electrode. Note that this diamond electrode is
not a multilayer structure. This electrode is composed of single layer
and basically homogeneous small grains of conductive diamond. Single
structure layer have the advantage that can be more easily produced in
CVD coating than multilayer coatings. Multilayer structure requires
change in the CVD parameter during the coating increasing the complexity
of process. Also the meaning of homogeneous small grains used in this
application do not means that the sizes of all grains are exactly the
same. It means that the small grains with size between 0.1-800 nm in
width; preferable in the range between 1-500 nm; more preferable in the
range between 1-300 nm are dispersed homogenously in the layer.

[0017]However, substrate coating with small diamond grains is a necessary
condition but not enough condition to obtain a high stability electrode.
As disclosed in the previous art, diamond layer with small grain
structure can be easily produced by increasing the concentration of
methane in the CVD chamber during the coating process. For example, if
methane concentration higher than 2% is used, there is a deposition of
small grain over the substrate material. Also if low substrate
temperature is used in the coating, for example at 650° C., the
obtained layer will be composed of small grain sizes, specifically
speaking with grain size smaller than one micrometer.

[0018]The present inventor have coated substrate at such CVD condition and
tested the produced diamond electrode in an electrolytic reaction. Detail
will be described after in the comparative examples, but diamond
electrode having small grain size layer produced by high methane
concentration or low CVD temperature, clearly fail in short time during
the electrolytic reaction. The reason is that the produced diamond layer
has a very poor diamond quality. Huge amount of non-diamond sp2
carbon are incorporated in the layer resulting in a poor stability of
diamond electrode. Beside the small grain size, diamond quality is
another necessary requirement to obtain long term stable diamond
electrode. The quality of the diamond can be quantitatively analyzed by
the ratio between amount of sp3 and sp2 carbons in the layer.
Diamond quality measured by Raman spectrophotometer, from hereafter will
be referred as Raman quality. In the Raman spectra, the sp3 diamond
carbons appear as a sharp peak at 1333 cm-1 and non-diamond sp2
carbons as a broad peak around 1500 cm-1. Raman quality can be
calculated by the area ratio of these two peaks, and 100% Raman quality
is the case where the layer is composed of high purity diamond and only
the sp3 peak is detected. Usually, the conductive diamond layer
produced by CVD process has Raman quality lower than 100%. CVD coating
with high methane concentration or low substrate temperature may easily
result in diamond layer with low Raman quality, and that are not a good
embodiment to produce long term stable diamond electrode.

[0019]In the present application, the value of Raman quality (q) is
calculated by the following equation (1) and its unit is given in
percentage. Id is the area of the diamond phase and Ind is the
area of the non diamond phases in the Raman graph.

q = 75 ∫ I d 75 ∫ I d + nd
∫ I nd 100 % ( 1 ) ##EQU00001##

This quantification of diamond quality can be done by the analysis of
diamond layer with a Raman spectrophotometer Type Ramanscope 2000 from
Renishaw. This spectrophotometer has a Argon laser with a wavelength of
514.5 nm and a lateral resolution of 1 μm. The measured area at a
magnification of 200× was ca. 25 μm. The values of Raman quality
used in this application refer to the calculated by above equation and
technique, but other techniques or devices can be used for the
quantification of diamond quality. In the case that other techniques are
used, even for the same diamond coating, some times different values can
be found.

[0020]According to the embodiment of this invention, Raman quality higher
than 50% is another required condition to provide a stable diamond
electrode. Raman quality lower than 50% means that sp2 carbon is
present in a detrimental amount in the diamond layer. Please, note that
if other techniques rather than described by equation (1) is used,
different values can be obtained. The important feature of this invention
is that the proportion of sp3 and sp2 carbon stays in a certain
range and when measured by the equation (1), it gives a value higher than
50%.

[0021]According to the embodiment of this invention, the Raman quality is
kept at value higher than 50% and at the same time providing a diamond
layer with small grains. Such a feature is achieved by coating the
substrate material in the CVD process in a controlled pressure. During
the CVD coating, the pressure is kept at value lower than 20 mBar (2 kPa)
but higher than 0.01 mBar (1 Pa), preferable at pressure between 15 mBar
(1.5 kPa) and 0.1 mBar (10 Pa) and further preferable between 6 mBar (600
Pa) and 1 mBar (100 Pa). The best range for producing a layer with small
grain and high Raman quality is when the pressure is between 1 mBar and 6
mBars. If the CVD chamber pressure becomes higher than 20 mBar, the grain
size of diamond will become larger facilitating the occurrence of
pinholes and other defects in the layer. This low CVD coating pressure
adopted in the present invention promotes the secondary nucleation of
diamond grains rather than increasing the grain size. This secondary
nucleation is promoted during the coating as low as is the CVD pressure.
There is a continuous formation of small grain nuclei at low CVD
pressure. But on the other hand if the pressure is too low, the absolute
density of methane (carbon source) inside CVD chamber becomes also low
and the growing rate of diamond layer will also become low. Furthermore,
in Hot Filament CVD, if the pressure is too low, there is a problem that
sparks can appear at the filament. The sparks can be prevented with an
adjusted process setup but if it happens, it will lead to the stop of the
coating process. From this reason, the pressure should be higher 0.01
mBar (1 Pa), preferable higher than 0.1 mBar (10 Pa) and further
preferable higher than 1 mBar (100 Pa). Therefore, according to the
embodiment of this invention, stable diamond electrode composed of small
grain size with Raman quality higher than 50% is provided and also the
method for producing such diamond electrode with a controlled pressure
lower than 20 mBar and higher than 0.01 mBar is provided. The above
pressures ranges are valid for Hot Filament CVD and Microwave CVD, when
producing diamond layer with small grains size.

[0022]Note that, the use of this low CVD pressure is a separate parameter
from the methane and hydrogen ratio. The balance of methane concentration
and Hydrogen concentration inside the CVD chamber is one important
parameter to control the Raman quality. Non-diamond sp2 carbon will
increase in the diamond layer, as high as is the methane concentration in
relation to the hydrogen concentration, because the relative value of
hydrogen radical that removes the sp2 carbon from the layer will
become low. The amount of hydrogen radical in the CVD chamber has to be
in a higher or at least stoichiometric amount to react with the sp2
carbons formed. For this reason, the concentration of methane in the CVD
chamber should be kept at value lower than 2% in relation to the hydrogen
gas, but not lower than 0.1%. If the methane concentration becomes lower
than 0.1%, also the growing rate of the diamond layer will decrease due
to the low absolute amount of the carbon source for sp3 carbon
formation.

[0023]Therefore, in this invention the grain size of diamond crystals is
controlled by the CVD pressure and the diamond quality is controlled by
other CVD parameter such as the methane concentration. That means this
invention provides a method for producing diamond layer with small grains
but without committing the diamond quality.

[0024]Accordingly, this invention provides a method for producing diamond
electrode by coating a substrate material by CVD process; said diamond
electrode having a single and homogeneous layer composed of a
poly-crystalline and conductive diamond with grain size lower than one
micrometer; said layer having a Raman quality higher than 50%; wherein
the said layer is produced by controlling the CVD at pressure lower than
20 mBar; and at methane concentration lower than 2%.

[0025]Such features are essential to produce a diamond electrode with
improved stability.

[0026]Another embodiment of present invention is related to the method for
producing the diamond electrode, in which the CVD coating is preceded by
a pretreatment step. Such pretreatment step comprises the seeding of
substrate with diamond nano crystal. The seed diamonds are important to
increase the growing rate of diamond layer during the CVD coating. If
there are not any diamond crystals that can work as the nuclei to start
the deposition of diamond carbons over the substrate, long coating time
will be required. The seed diamond can be impregnated in the substrate by
immersing the substrate in a solution containing seed diamond, water and
some solvent such as methanol, ethanol or acetone. This impregnation of
seed diamond is preferable done in a bath where there is an ultra-sonic
treatment. The seed diamonds can not be higher than one micrometer, by
obvious reason, if this invention intents to provide a homogeneous layer
composed of diamond grains lower than one micrometer. However, the seed
diamonds are preferable lower than 200 nm, more preferable lower than 50
nm, further preferable lower than 5 nm. These nano seed crystals are
necessary for providing many connection points between the substrate and
the diamond layer in order to improve the cohesion of the coating.
Furthermore the nano seed crystals reduce the process time until a dense
diamond layer is grown by the coalescence of the seed crystals.

[0027]In another embodiment of the present invention, a method for
producing the diamond electrode, wherein the diamond layer has a
thickness of at least one micrometer is provided. The grain size that
composes the layer should be small, with size between 0.1-800 nm in
width; preferable in the range between 1-500 nm; more preferable in the
range between 1-300 nm. However, if the layer thickness is too thin, the
probability of electrolyte solution infiltrate in the layer will
increase. By this reason, for a long term stable diamond electrode, the
layer thickness should be at least of one micrometer, more preferable
higher than 5 micrometer; further preferable if higher than 10
micrometer.

[0028]This invention also provides a method for producing the diamond
electrode, wherein the diamond layer has a boron doping level lower than
1,500 ppm (part per million). The doping level here, refers to the molar
ratio between boron and carbon (B/C ratio) in the layer. As high as is
the B/C ratio, the electrical conductivity of diamond layer will
increase. From the point of view of diamond electrode application, this
conductivity has some benefits because it can decrease the voltage
between the electrodes during the electrochemical reaction. On the other
hand, the boron induces the deposition of sp2 (non-diamond)carbons
in the layer during the CVD coating. When the B/C ratio is higher than
1,500 ppm the amount of sp2 carbons will be in a detrimental amount
inside of the layer. The Raman quality of the layer will decrease with
the increase in the B/C ratio. For this reason the doping level should be
low than 1,500 ppm.

[0029]Additionally, this invention provides a method for producing the
diamond electrode, wherein the coating is performed in a hot-filament CVD
with the filament disposed vertically inside of the CVD chamber. If the
filaments are disposed horizontally, there will be a slackening of the
filament during the CVD coating due to the thermal expansion of filament
wires and due to the gravity. The distance between the filaments and/or
between the filament and substrate can not be kept uniform. The
slackening of filament tends to occurs because the filament achieves a
temperature of 1,800 to 2,400° C. during the HF-CVD coating. The
distance between the filaments and/or between the filament and substrate
shall be kept in a prescribed value to achieve a homogeneous coating in
the whole substrate surface. The slackening of filament do not occur when
disposed vertically because the gravity works to stretching the filament
wires.

BRIEF DESCRIPTION OF DRAWINGS

[0030]FIG. 1 is a schematic illustration of delamination mechanism
originated by pinhole when the diamond layer are composed of grains
larger one micrometer; FIG. 1a shows a short path for the penetration of
electrolyte solution and FIG. 1b shows the subsequent delamination caused
by this penetration of electrolyte solution.

[0031]FIG. 2 is a schematic illustration of diamond electrodes composed of
under-micrometer particles where the attack of electrolyte solution to
the interlayer is suppressed by its long penetration path.

[0032]FIG. 3 is a schematic illustration of hot filament CVD process for
coating the diamond electrodes, wherein the filament 2 is disposed
vertically.

[0033]FIG. 4 is a Scanning Electronic Microscope (SEM) picture of the
diamond electrode surface produced at CVD pressure of 20 mBar according
to the Comparative Ex. 1

[0034]FIG. 5 is a Scanning Electronic Microscope (SEM) picture of the
diamond electrode surface produced at CVD pressure of 6 mBar according to
the Example 1.

[0035]FIG. 6 is a schematic illustration of the electrolytic cell used for
the stability evaluation of produced diamond electrode.

[0036]FIG. 7 is the profile of electrode voltage and current density in
function of the electrical charge density per micrometer of electrode
thickness during the electrolytic test of diamond electrode produced at
20 mBars and according to the Comparative Ex. 1.

[0037]FIG. 8 is the profile of electrode voltage and current density in
function of the electrical charge density per micrometer of electrode
thickness during the electrolytic test of diamond electrode produced at 6
mBars and according to the Example 1.

[0038]FIG. 9 is the profile of electrode voltage and current density in
function of the electrical charge density per micrometer of electrode
thickness during the electrolytic test of diamond electrode produced at
15 mBars and according to the Example 2.

[0039]FIG. 10 is a Scanning Electronic Microscope (SEM) picture of the
diamond electrode surface produced at CVD pressure of 6 mBar and methane
concentration of 2% according to the Comparative Ex. 2.

[0040]FIG. 11 is the profile of electrode voltage and current density in
function of the electrical charge density per micrometer of electrode
thickness during the electrolytic test of diamond electrode produced at 6
mBars and methane concentration of 2% according to the COMPARATIVE Ex. 2.

[0041]FIG. 12 is a Scanning Electronic Microscope (SEM) picture showing
the growing behavior of diamond crystal over the substrate material (1.5
hour CVD coating) when the pretreatment was done by seed diamond with
average size of 250 nm and according to the Comparative Ex. 3.

[0042]FIG. 13 is a Scanning Electronic Microscope (SEM) picture showing
the growing behavior of diamond crystal over the substrate material (1.5
hour CVD coating) when the pretreatment was done by seed diamond with
average size of 5 nm and according to the Example 3.

BEST MODE FOR CARRYING OUT THE INVENTION

[0043]Hereinafter, the embodiment of the present invention will be
described in detail and with reference to the COMPARATIVE EX. and
EXAMPLES. FIG. 3 illustrates the basic configuration of a HF CVD
apparatus that was used for the diamond coating in the COMPARATIVE Ex.
and EXAMPLES described hereinafter. However, for the execution of
features of present invention, the CVD apparatus are not limited to this
one illustrated in this FIG. 3 and can be as well as performed in
Micro-wave Plasma CVD. The HF CVD apparatus is comprised by a CVD chamber
1 and a filament 2 disposed inside and vertically. The CVD chamber is a
sealed chamber in which the pressure can be kept lower than atmospheric
pressure. The control of pressure is achieved by means of vacuum pump 7.
Also line 4, line 5 and line 6 are provided to supply the hydrogen,
carbon source and a dopant source, respectively. The line 4, line 5 and
Line 6 are connected to a mass flow controller (not shown) to keep the
respective gases at certain concentration inside the CVD chamber. For an
accurate control of the pressure inside of the chamber, a valve for the
control of suction rate can be disposed in the line between the chamber
and the vacuum pump. The flow rates, including the flow rate of outlet
gases and inlet gases in the CVD chamber, can be electronically
controlled by automatic systems using computer processors. The substrate
3, which will be coated, is disposed in front of the filament 2. During
the coating, the filament is heated at temperature between
1,800-2,400° C. by supplying a direct current to the filament 2.
The substrate temperature can be kept at temperature between
700-900° C. by the irradiation of the filament 2. Additional
devices for the adjustment of substrate temperature can be used. For
example heater can be disposed behind the substrate for this purpose.
COMPARATIVE EX. 1 and EXAMPLE 1 show that the CVD pressure can control
the grain size of diamond. Comparative example 1 was coated at 20 mBar
and Example 1 was coated at CVD pressure of 6 mBar.

EXAMPLES

Comparative Ex. 1

[0044]The surface of titanium plate (40×60×4t) was pretreated
by sand-blasting using SiC powders as the blasting material. The
sand-blasted titanium plate, after washing with distilled water, was
immersed in an ultra-sonic bath containing aqueous ethanol solution and
seed diamond with diameter around 5 nm. The substrate material was
treated in this ultrasonic-bath for 10 h. After drying, the substrate
material was placed inside the HF-CVD chamber and coated at 20 mBar and
at the condition illustrated in TABLE 1 for 20 h.

[0045]The produced electrode had a diamond layer of 1.7 μm. FIG. 4
illustrates a SEM picture of the produced diamond layer. A lot of grains
are larger than one micrometer. In average, the grains produced by
coating at 20 mBar are larger.

[0046]The stability of diamond electrode was tested in an electrochemical
cell as illustrated in FIG. 6. The direct current was supplied to the
electrode by a DC-FEED 8. The DC-FEED is connected to Anode 9 and the
Cathode 10. The diamond electrode of COMPARATIVE EX. 1 was used as the
anode and a titanium plate was used as the cathode.

[0047]The testing electrolyte solution 11 was composed of aqueous solution
containing 20 g per litter of acetic acid and 0.1M of sodium sulfate as
supporting electrolyte. The electrolyte solution was filled in a glass
beaker 12. During all the test period, the solution was stirred by means
of a magnetic mixer 13 and a stirrer 14. The gap between the electrodes
was kept at 4 mm.

[0048]The electrochemical cell was operated at a galvanostatic condition,
that means, the DC-Feed 8 was operated at constant current and the
electrode was controlled at constant current density of 150 mA/cm2.

[0049]FIG. 7 illustrates the profile of voltage between the electrodes
(left vertical axis) and the current density (right vertical axis) in
function of the electrical charge density per micrometer of diamond layer
thickness (horizontal axis). The electrical charge density per micrometer
of diamond layer thickness, hereinafter referred as charge density, which
the unit is given in Ah/(cm2μm), indicates the amount of
electrical charge (Ah) that was passed in a square centimeter of
electrode area divided by the thickness of diamond layer. As high is this
value, higher will be operation time of electrode taking into the account
the current density and thickness of diamond layer. Accordingly, this
charge density is a good reference to evaluate the stability of the
electrode.

[0050]The electrode produced in COMPARATIVE Ex. 1, as can be seen in FIG.
7, after passing a charge density of roughly 10 Ah/(cm2μm) shows
an increase in the voltage between the electrodes. This increase in the
voltage is due to decrease of effective area of diamond electrode by the
delamination of diamond layers. Between the diamond layer and the
titanium substrate, there is a thin interlayer of titanium carbide. But
because this titanium carbide layer is not chemically stable against the
attack of electrolyte solution, the titanium carbide layer quickly
dissolves exposing the titanium substrate. After the delamination, the
exposed titanium substrates are passivated by the formation of titanium
oxide layer which is a ceramic form of titanium and electrically not
conductive. Then, the delaminated area becomes electrically not
conductive, causing the increase in the voltage between the electrodes.
This point where the electrode voltage starts to increase is used as a
reference for the charge density where the electrode starts to fail by
delamination. When further continuing the electrochemical reaction, there
is a point where the reaction can not be kept more at galvanostatic
condition because the voltage between the electrodes achieves the maximum
capacity that the DC-supply can supply. In the case of COMPARATIVE and
EXAMPLES described in this application, the used DC-Feed had a maximum
capacity of 20V. In COMPARATIVE EX. 1, the current density started to
decrease after passing a charge density of 14 Ah/(cm2μm). After
passing a charge density of around 20 Ah/(cm2μm), this electrode
was found to be completely failed with the whole surface delaminated, and
current density decreasing to values near zero.

Example 1

[0051]The surface of titanium plate (40×60×4t) was pretreated
by sand-blasting using SiC powder as the blasting material. The
sand-blasted titanium plate after washing with distilled water, was
immersed in an ultra-sonic bath containing aqueous ethanol solution and
seed diamond with diameter around 5 nm. The substrate material was
treated in this ultrasonic-bath for 10 h. After drying, the substrate
material was placed inside the HF-CVD chamber and coated at 6 mBar and at
the condition illustrated in TABLE 1 for 20 h.

[0052]The produced electrode had a diamond layer of 1.35 μm. FIG. 5
illustrates a SEM picture of the diamond electrode surface produced in
EXAMPLE 1. As can be seen, the grains of diamond crystal are very small
and this is due to fact that this electrode was coated at CVD pressure of
6 mBar. The grain sizes are lower than one micrometer, which can be
confirmed by the reference bar of 2 μm illustrated in FIG. 5.
Comparing with FIG. 4 where the coating was performed at CVD pressure of
20 mBar, the grains produced by coating at CVD pressure of 6 mBar, as
illustrated in FIG. 5, are clearly small, proofing that CVD pressure is
one important parameter that can control the crystal size of diamond
layer. An AFM (atomic force microscopy) analysis showed that the grain
size in the EXAMPLE 1 had a grain size of around 300 nm.

[0053]The stability of diamond electrode was tested in an electrochemical
cell as illustrated in FIG. 6. The direct current was supplied to the
electrode by a DC-FEED 8. The DC-FEED was connected to Anode 9 and the
Cathode 10. The diamond electrode of EXAMPLE 1 was used as the anode and
a titanium plate was used as the cathode.

[0054]The testing electrolyte solution 11 was composed of aqueous solution
containing 20 g per litter of acetic acid and 0.1M of sodium sulfate as
supporting electrolyte. The electrolyte solution was filled in a glass
beaker 12. During all the test period, the solution was stirred by means
of a magnetic mixer 13 and a stirrer 14. The gap between the electrodes
was kept at 4 mm.

[0055]The electrochemical cell was operated at a galvanostatic condition,
that means, the DC-Feed 8 was operated at constant current and the
electrode was controlled at constant current density of 150 mA/cm2.

[0056]FIG. 8 illustrates the profile of voltage between the electrodes
(left vertical axis) and the current density (right vertical axis) in
function of the electrical charge density per micrometer of diamond layer
thickness (horizontal axis) for the electrode produced in EXAMPLE 1.

[0057]The voltage between the electrode in EXAMPLE 1 started to increase
only when passing a charge density higher than 20 Ah/(cm2μm). The
value of charge density where the electrode voltage started to increase
in COMPARATIVE EX. 1 (FIG. 7) was 10 Ah/(cm2μm). That means the
electrode of EXAMPLE 1 started to delaminate only when passing the double
of charge density compared to the electrode of COMPARATIVE EX. 1. Also
referring to the value of Raman quality illustrated in Table 1, it is
clear that the electrode of COMPARATIVE Ex. 1 has higher Raman quality
(76.7%) rather than the electrode of EXAMPLE 1 (52%). The amount of
sp3 carbon and the diamond quality in the layer are higher in
COMPARATIVE EX. 1, rather than in EXAMPLE 1. Even with this advantage in
the Raman quality, the electrode of COMPARATIVE EX. 1 started to fail by
delamination prior to the electrode of EXAMPLE 1. That is due to the fact
that the grain size of diamond layer in EXAMPLE 1 has small grain
structure which avoids or makes difficult the penetration of electrolyte
solution by the mechanism illustrated in FIG. 2.

[0058]The electrode of EXAMPLE 1 achieved a voltage of 20V after passing a
charge density of 26 Ah/(cm2μm) and the following continuation of
electrolytic solution had a decrease in current density. Note that in
COMPARATIVE EX. 1 the current density started to decrease at 14
Ah/(cm2μm), showing that the stability of electrode in EXAMPLE 1
is clearly better than the electrode of COMPARATIVE EX. 1.

Example 2

[0059]The surface of titanium plate (40×60×4t) was pretreated
by sand-blasting using SiC powder as the blasting material. The sand
blasted titanium plate, after washing with distilled water, was immersed
in an ultra-sonic bath containing aqueous ethanol solution and seed
diamond with diameter around 5 nm. The substrate material was treated in
this ultrasonic-bath for 10 h. After drying, the substrate material was
placed inside the HF-CVD chamber and coated at 15 mBar and at the
condition illustrated in TABLE 1 for 20 h in total. Here, the electrode
was coated by 10 h using methane concentration of 1.3% and at the
following 10 h the methane was changed to 0.8%.

[0060]The produced electrode had a diamond layer of 1.7 μm. The Raman
quality of the produced layer was 78.5% showing that the decrease in
methane concentration during the CVD coating can increase the Raman
quality. The grain sizes were lower than one micrometer, with an average
size of 700 nm confirmed by SEM and AFM analysis. The grains sizes were
lower than that one produced at CVD pressure of 20 mBar and illustrated
in FIG. 4 (COMPARATIVE EX. 1).

[0061]The stability of diamond electrode was tested in an electrochemical
cell as illustrated in FIG. 6. The direct current was supplied to the
electrode by a DC-FEED 8. The DC-FEED was connected to Anode 9 and the
Cathode 10. The diamond electrode of EXAMPLE 2 was used as the anode and
a titanium plate was used as the cathode.

[0062]The testing electrolyte solution 11 was composed of aqueous solution
containing 20 g per litter of acetic acid and 0.1M of sodium sulfate as
supporting electrolyte. The electrolyte solution was filled in a glass
beaker 12. During all the test period, the solution was stirred by means
of a magnetic mixer 13 and a stirrer 14. The gap between the electrodes
was kept at 4 mm.

[0063]The electrochemical cell was operated at a galvanostatic condition,
that means, the DC-Feed 8 was operated at constant current and the
electrode was controlled at constant current density of 150 mA/cm2.

[0064]FIG. 9 illustrates the profile of voltage between the electrodes
(left vertical axis) and the current density (right vertical axis) in
function of the electrical charge density per micrometer of diamond layer
thickness (horizontal axis) for the electrode produced in EXAMPLE 2.

[0065]The voltage between the electrode in EXAMPLE 2 started to increase
only when passing a charge density higher than 24 Ah/(cm2μm). The
value of charge density where the electrode voltage started to increase
in COMPARATIVE EX. 1 (FIG. 7) was 10 Ah/(cm2μm). In comparison,
the electrode of EXAMPLE 2 started to delaminate only when passing more
than the double of charge density. The value of Raman quality and diamond
layer thickness has almost the same values when comparing EXAMPLE 2 and
COMPARATIVE EX. 1, as illustrated in Table 1. However, stability of
electrode in EXAMPLE 2 was more than the double of that one in
COMPARATIVE EX. 1. This is due to the fact that the grain size of diamond
layer in EXAMPLE 2 has small grain structure which avoids or makes
difficult the penetration of electrolyte solution by the mechanism
illustrated in FIG. 2.

[0066]The electrode of EXAMPLE 2 achieved a voltage of 20V after passing a
charge density of 35 Ah/(cm2μm) and the following continuation of
electrolytic solution caused a decrease in current density. Note that in
COMPARATIVE EX. 1 the current density started to decrease at 14
Ah/(cm2μm), showing that the stability of electrode in EXAMPLE 2
is clearly better than the electrode of COMPARATIVE EX. 1. Also comparing
the EXAMPLE 1 and EXAMPLE 2 (see FIG. 8 and FIG. 9), the stability was
better in EXAMPLE 2, due to the higher thickness and higher Raman quality
(see Table 1).

Comparative Ex. 2

[0067]The surface of titanium plate (40×60×4t) was pretreated
by sand-blasting using SiC powders as the blasting material. The
pre-treated titanium plate, after washing with distilled water, was
immersed in an ultra-sonic bath containing aqueous ethanol solution and
seed diamond with diameter around 5 nm. The substrate material was
treated in this ultrasonic-bath for 10 h. After drying, the substrate
material was placed inside the HF-CVD chamber and coated at 6 mBar with
methane concentration of 2% and at the condition illustrated in TABLE 1
for 20 h.

[0068]The produced electrode had a diamond layer of 1.7 μm. FIG. 10
illustrates a SEM picture of the produced diamond layer. The grains sizes
are very small ranging around 100 nm, confirmed by AFM analysis. This
small grain size is a result of the CVD coating at low pressure and high
methane concentration. This demonstrates that the use of low pressure as
well as high methane concentration can decrease the size of grains in the
diamond layer. The stability of this diamond electrode was tested in an
electrochemical cell as illustrated in FIG. 6. The direct current was
supplied to the electrode by a DC-FEED 8. The DC-FEED is connected to
Anode 9 and the Cathode 10. The diamond electrode of COMPARATIVE EX. 2
was used as the anode and a titanium plate was used as the cathode.

[0069]The testing electrolyte solution 11 was composed of aqueous solution
containing 20 g per litter of acetic acid and 0.1M of sodium sulfate as
supporting electrolyte. The electrolyte solution was filled in a glass
beaker 12. During all the test period, the solution was stirred by means
of a magnetic mixer 13 and a stirrer 14. The gap between the electrodes
was kept at 4 mm.

[0070]The electrochemical cell was operated at a galvanostatic condition,
that means, the DC-Feed 8 was operated at constant current and the
electrode was controlled at constant current density of 150 mA/cm2.

[0071]FIG. 11 illustrates the profile of voltage between the electrodes
(left vertical axis) and the current density (right vertical axis) in
function of the electrical charge density per micrometer of diamond layer
thickness (horizontal axis) for the electrode produced in COMPARATIVE Ex.
2.

[0072]The voltage between the electrode in COMPARATIVE EX. 2 started to
increase after passing a charge density of 15 Ah/(cm2μm). The
value of charge density where the electrode voltage started to increase
in COMPARATIVE EX. 2 was clearly low than EXAMPLE 1 and EXAMPLE 2.
Despite the diamond layer of COMPARATIVE Ex. 2 is composed of small grain
size, the stability are lower than EXAMPLE 1 and EXAMPLE 2. This is due
to the low Raman quality of this electrode as can be seen in Table 1. The
Raman quality in this COMPARATIVE EX. 2 was 38.5%, and this low diamond
quality is because this electrode was coated at a methane concentration
of 2%. This shows that, when increasing the methane concentration during
the CVD coating, it is possible to produce layer with fine particle.
However, because the quality of diamond layer produced at this condition
is low, the stability of electrode is also low. To have a stable diamond
electrode, the small grain size is not condition enough. Also the Raman
quality is another requirement. The Raman quality of the layer has to be
higher than 50%, to obtaining a stable diamond electrode. In this case,
the factor that is decreasing the Raman quality is the high methane
concentration. Methane concentration of 2% allows to obtaining layer
composed of small particle but it also causes the decrease of quality of
diamond layer. In other words, to improve the stability of diamond
electrodes, it is preferable to make the coating using methane
concentration lower than 2%.

[0073]The electrode of COMPARATIVE Ex. 2, after passing a charge density
of 22 Ah/(cm2μm) achieve 20V during the electrolytic test. In
further continuation of the test, the galvanostatic condition could not
be kept, with decrease in current density.

[0074]In all Comparative Ex. and Examples, while the electrode did not
fail, the Acetic Acid in the electrolyte solution was degraded to carbon
dioxide and water during the electrolytic test. The degradation of Acetic
acid was monitored by COD (Chemical Oxygen Demand) measurement and there
was a clear decrease of COD during the electrolytic test. The COD
decreased at a rate of near 3.35 Ah/g-COD. This decrease in COD was due
to the oxidation of acetic acid by the OH radical produced at the diamond
electrode.

Example 3 and Comparative Ex. 3

[0075]Example 3 and Comparative Ex. 3 illustrate the influence of the size
of seed diamonds in the growth behavior of diamond crystal during the CVD
coating. Surface of two titanium plate (40×60×4t) were
pretreated by sand-blasting using SiC powders as the blasting material.
The pre-treated titanium plates, after washing with distilled water, were
immersed in an ultra-sonic bath containing aqueous ethanol solution and
seed diamond. The seed diamond used for Example 3 and Comparative Ex. 3
have an average diameter of 5 nm and 250 nm respectively. The substrates
were treated in this ultrasonic-bath for 10 h. After drying, the
substrates were placed inside the HF-CVD chamber and coated at 6 mBar
with methane concentration of 1.3%. The coating was interrupted after 1.5
h to see the difference of growing behavior of the diamond crystal over
the substrate. The CVD condition for Comparative Ex. 3 and Example 3, as
illustrated in Table 1, were exactly the same except in the difference in
the size of seed diamond during the pretreatment. The Raman quality and
the layer thickness were not measured (nm) in Comparative Ex. 3 and
Example 3, because due to the short time coating. The coating time was
not enough for the formation of a layer over the substrate. FIG. 12 and
FIG. 13 illustrates the SEM picture after the CVD coating for Comparative
Ex. 3 and Example 3, respectively. The white small points in this picture
are the diamond crystal. As can be seen in FIG. 12, when diamond of 250
nm was used as the seed, the number of diamond crystal that can be
recognized by the SEM are scattered and in low quantity. In comparison,
as can be seen in FIG. 13, when diamond size of 5 nm is used as the seed,
a lot of small crystal can be recognized over whole surface of substrate.
Therefore, it is clear that when nano-sized diamonds are used as the
seed, the formation of a dense diamond layer over the substrate will be
faster than large seed. The process time until a dense diamond layer is
grown to the whole substrate surface by the coalescence of the seed
crystals can be shortened using seeds of nano diamonds. The size of seed
nano diamonds are preferable lower than 200 nm, more preferable lower
than 50 nm and further preferable when lower than 5 nm.

INDUSTRIAL APPLICABILITY

[0076]A method for production of diamond electrodes with improved
stability is provided in the present invention. In the present invention,
a diamond electrode, having at least one poly-crystalline and conductive
diamond layer, the layer having grain size lower than one micrometer with
Raman quality higher than 50%, is coated by a CVD process controlling the
pressure to lower than 20 mBar.